ipv6: release dst properly in ipip6_tunnel_xmit

if a dst is not attached to anywhere, it should be released before
exit ipip6_tunnel_xmit, otherwise cause dst memory leakage.

Fixes: 61c1db7fae21 ("ipv6: sit: add GSO/TSO support")
Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>